1. Field of the Invention
The present invention relates to a method for continuous copper electroplating of a workpiece to be plated by use of a copper sulfate plating bath.
2. Description of the Related Art
In the electroplating conducted using an insoluble anode together with a copper sulfate plating bath, the absence of dissolution of copper from the anode results in that the copper concentration in the plating bath decreases as the electroplating proceeds. In order to restore the lowered copper concentration to a level suitable for copper plating, therefore, a copper salt such as copper oxide, copper carbonate, and copper hydrate is supplied as a copper ion supplying salt.
However, those of the copper ion supplying salts which are high in purity are expensive. In many cases, therefore, inexpensive copper ion supplying salts which contain organic impurities or the like, such as the copper oxide recycled from waste etching solutions, are used. Besides, since the reactions for dissolving the copper ion supplying salts may be exothermic, additives present in the plating bath may be denatured during the reactions. Therefore, the use of such copper ion supplying salts may exert adverse effects on the plating; specifically, it may lead to generation of defective appearance or to degradation in throwing power and/or hole filling (plugging) performance.
To cope with the bad influences on plating, the plating performance impairing components (the components detrimental to plating performance) have hitherto been removed by treating the plating bath with active carbon. Or, alternatively, air bubbling has been applied to metallic copper in the plating bath to effect oxidation and decomposition, thereby converting the plating performance impairing components into components which are not detrimental to plating. Particularly, the active carbon treatment has the problem of lowered productivity, since it is necessary to perform the treatment periodically by stopping the plating process, for removing the plating performance impairing components.
As a method for supplying copper ions into a plating bath, for example, JP-A 2000-109998 discloses a plating method in which an anode chamber provided near an insoluble anode is used. This method, however, has the following problem. In the method, the transport of the plating bath to and from the dissolution vessel is carried out on the anode side in the anode chamber isolated by a membrane which substantially is impermeable to the plating bath. Particularly in the case where the current density is high and the metal dissolution cycle is rapid, therefore, the rate of migration of metallic ions from the anode chamber to the cathode is controlled (limited) by permeation through the membrane. Consequently, the supply of metallic ions to the cathode is delayed, and the metallic ion concentration in the vicinity of the cathode would be lowered. Thus, it is difficult to maintain good plating quality.
Besides, JP-T Hei 8-507106 discloses a method and apparatus for electrolytic deposition of a metal layer. In the method and apparatus, however, the plating bath returned from a metallic ion generator is supplied directly to the cathode, so that plating performance lowering components derived from the metallic ion supplying salts are supplied directly to the cathode, to lower the plating performance. Also, in the copper sulfate plating method described in Japanese Patent No. 3903120, there is the same problem as above because the plating bath is returned from the copper oxide dissolution vessel to the cathode side.
Incidentally, in addition to the above-mentioned patent documents, JP-A 2005-187869 is cited as a prior-art reference.